In drilling wells for oil and gas exploration, understanding the structure and properties of the associated geological formation provides information to aid such exploration. A number of different measurements in a borehole can be performed to attain this understanding. Measurements may include using a neutron tool to make measurements to determine properties of a formation under investigation. Further, the usefulness, efficiency, and accuracy of traditional measurements may be related to the precision or quality of the techniques to attain and process data derived from such measurements. Techniques and apparatus to simplify measurements, to enhance processing of measured data, to enhance analysis of data from measurements to provide properties of a formation or borehole, or to provide combinations thereof can further aid in drilling operations.
In some conventional tools, porosity is measured using a pulsed-neutron generator and two detectors that measure gamma rays, which can be presented as count rates of gamma rays. One of the detectors is spaced from the pulsed-neutron generator at a larger distance than the other detector, defining near and far detectors. The ratio of the near-to-far count rates of gamma rays measured between 130 and 1000 μsec after the neutron burst can be generated in an example tool. Such a tool can have a problem in measuring porosity in cased holes at high porosity, because the tool response may be insensitive to porosity at high porosities. This is illustrated in FIG. 1, where a near/far ratio is shown for data obtained from computer modeling. The model was generated for limestone formations filled with fresh water and a 7-in casing with a 0.25-inch wall thickness and 1.4-inch thick cement. The ratio in this case has very little sensitivity to porosity above 30 p.u.